iwlwifi: fix build error for CONFIG_IWLAGN=n

drivers/net/wireless/iwlwifi/iwl-rx.c: In function 'iwl_good_ack_health':
drivers/net/wireless/iwlwifi/iwl-rx.c:647: error: 'struct iwl_priv' has no member named '_agn'
Reported-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/iwlwifi/iwl-rx.c

@@ -621,6 +621,7 @@ static void iwl_accumulative_statistics(struct iwl_priv *priv,
 #define BA_TIMEOUT_CNT (5)
 #define BA_TIMEOUT_MAX (16)
 
+#if defined(CONFIG_IWLAGN) || defined(CONFIG_IWLAGN_MODULE)
 /**
  * iwl_good_ack_health - checks for ACK count ratios, BA timeout retries.
  *
@@ -670,6 +671,7 @@ bool iwl_good_ack_health(struct iwl_priv *priv,
 	return rc;
 }
 EXPORT_SYMBOL(iwl_good_ack_health);
+#endif
 
 /**
  * iwl_good_plcp_health - checks for plcp error.